Roof structures must display a plurality of features meeting diverse criteria. The roof must be strong enough to withstand a load, particularly a snow load in cold climates, or traffic for service or repair. Roof structures also must be waterproof, and in most climates they must be insulating. Insulation is required in cold climates to prevent condensation on the underside of the roof structure, and in warm climates insulation is necessary to prevent entrance of heat which would render air conditioning less effective.
In conventional roofs, a deck of some sort is provided. This deck may be of wood, steel or concrete. Some sort of waterproofing materials such as roofing felts and layers of asphalt or bitumin may be applied for flat roofs, topped with something such as stone ballast to help hold down the sheet material, and also to protect the asphalt or bitumin from destructive ultraviolet rays of the sun. In pitched roofs shingles may be used. It is common practice to put some sort of insulation, such as bats of fiber glass beneath the deck.
An alternative construction known as protected membrane roofing has come into wide use in which a roof deck is provided with an overlying membrane with insulation placed on top of the membrane. This insulation is generally a closed cell plastic resin foam, of which polystyrene foam is a preferred example. Stone ballast or other protective material may be placed on top of the insulation. The insulation itself is characterized by having very little absorption of moisture. A certain amount of water does get through the joints between adjacent sections of insulation, and a small amount of water may be absorbed by the insulation. However, all of this dries out nicely on warm, dry days.
Protected membrane roofs have presented an unusual problem for certain installations. In high humidity buildings where the temperature is high, for example, 90 F., and where relative humidity is also high, on the order of 70%, such as in a paper mill, the dew point temperature is often close to the ambient inside air temperature at the ceiling. Sometimes the dew point temperature and interior temperatures are within 10 F. of one another, or perhaps even closer. Consequently, even a slight reduction in deck undersurface temperature will result in condensation on the deck underside. In a protected membrane roof in which the waterproofing membrane is provided on the deck and under the insulation, the membrane is in intimate contact with the roof deck. During cold rains the temperature of water passing between adjacent panels of insulation and contacting the membrane sometimes reduces the deck underside temperature (the deck typically being thin concrete with little mass) to where the dew point is reached, and condensation results. Condensation on the underside of the roof deck is undesirable as it causes a deterioration thereof, and furthermore it may drip in areas where such dripping cannot be tolerated.